An image sensor can be a semiconductor device that produces an electrical signal in response to incoming light. An image sensor can be, for example, a charge coupled device (CCD) or a complementary metal oxide silicon (CMOS) image sensor. A CCD can be complicated in operation and manufacturing and may have relatively large power consumption. Recently, CMOS image sensors have been used in various products such as digital cameras, camcorders, personal communication systems (PCS), and game machines.
An image sensor usually includes a pixel array, an analog-to-digital converter (ADC), a line memory, and a sense amplifier. The pixel array can include several tens of thousand to several millions of pixels arranged in a matrix. Each pixel in the array includes a photo sensitive device which senses incoming light. The photo sensitive device can convert incoming photons into electron-hole pairs, which can accumulate in the photosensitive device as charges. The amount of accumulated charge can vary depending on the intensity and the amount of incoming light. Each pixel measures associated accumulated charge and outputs an electrical analog signal.
The ADC can convert the electrical analog signal output from the pixel array into a digital signal. The digital signal can be temporarily stored in the line memory. The sense amplifier can amplify the digital signal transmitted from the line memory.
The operating speed of an image sensor can be directly related to the digital signal readout speed, which may depend on the time needed for the digital signal stored in the line memory to be transmitted to the sense amplifier (via the bus line), amplified and then output by the sense amplifier. When the time taken for this process decreases, the operating speed of the image sensor can be increased. One of the aspects influencing the speed at which the digital signal can be generated is the parasitic resistance and capacitance associated with the bus line. In other words, the digital signal readout speed may be affected by parasitic resistance and capacitance of the bus line.